Bump version to 5.0.0-alpha1 (#29925)

Also add v4.5.0 in versions and keep README.md pointing to v4.5.0 so that there are no broken stuff.

site/content/docs/5.0/examples/navbar-fixed/index.html

@@ -0,0 +1,40 @@
+---
+layout: examples
+title: Fixed top navbar example
+extra_css:
+  - "navbar-top-fixed.css"
+---
+
+<nav class="navbar navbar-expand-md navbar-dark fixed-top bg-dark">
+  <div class="container-fluid">
+    <a class="navbar-brand" href="#">Fixed navbar</a>
+    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
+      <span class="navbar-toggler-icon"></span>
+    </button>
+    <div class="collapse navbar-collapse" id="navbarCollapse">
+      <ul class="navbar-nav mr-auto mb-2 mb-md-0">
+        <li class="nav-item active">
+          <a class="nav-link" aria-current="page" href="#">Home</a>
+        </li>
+        <li class="nav-item">
+          <a class="nav-link" href="#">Link</a>
+        </li>
+        <li class="nav-item">
+          <a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a>
+        </li>
+      </ul>
+      <form class="d-flex">
+        <input class="form-control mr-2" type="search" placeholder="Search" aria-label="Search">
+        <button class="btn btn-outline-success" type="submit">Search</button>
+      </form>
+    </div>
+  </div>
+</nav>
+
+<main class="container">
+  <div class="bg-light p-5 rounded">
+    <h1>Navbar example</h1>
+    <p class="lead">This example is a quick exercise to illustrate how fixed to top navbar works. As you scroll, it will remain fixed to the top of your browser’s viewport.</p>
+    <a class="btn btn-lg btn-primary" href="{{< docsref "/components/navbar" >}}" role="button">View navbar docs &raquo;</a>
+  </div>
+</main>